An altitude-azimuth or “alt-azimuth” tracking system has two axes, a first axis that is vertical, about which the system rotates to a desired azimuth (or bearing) measured eastwards from north, and a second, horizontal axis (which itself rotates on the first axis), about which the system rotates to the desired altitude, i.e. angle above the horizon. With these two (vertical and horizontal) axes, the tracking system can point an instrument such as a telescope or solar power collector at any point above the horizon. By driving both axes in a suitable manner, that instrument can be held in alignment with the tracked object. This might comprise, for example, tracking the sun from sunrise to sunset.
An example of a background art solar energy collector with dish concentrator, mounted on an alt-azimuth tracking system and located in the southern hemisphere, is shown generally at 10 in FIG. 1. The dish concentrator 12 is located on a mount 14. The mount is supported in a yoke 16 which allows the mount 14 to rotate about horizontal axis 18. The yoke 16 is supported on a drum 20, rotatable about a vertical axis. The drum 20 is supported by a pylon 22. As illustrated, this arrangement 10 would be suitable for tracking the sun from the southern hemisphere and hence will more often than not be pointing (as shown) northwards.
However, if a tracked object passes directly overhead (viz. through the zenith) a problem can arise. For example, if the object is the sun, such an event will occur in the tropics (i.e. between latitude 23.5° north and 23.5° south) around two times of the year. On the equator, for example, this occurs at the vernal and autumnal equinoxes, that is, approximately 21 March and 21 September. As seen from the equator on the equinoxes, the sun rises due east (i.e. azimuth 90°, altitude 0°), and then sets due west (i.e. azimuth 270°, altitude 0°) essentially twelve hours later. For the first six hours, the altitude increases from 0 to 90° at 15° per hour, while for the second six hours the altitude decreases from 90° to 0° at the same rate. The azimuth remains at 90° for the first six hours and at 270° for the second six hours. To track the sun under these conditions, therefore, a conventional altitude-azimuth tracking system is required to rotate from azimuth 90° to azimuth 270° when the sun reaches zenith, essentially instantaneously. This, as will be appreciated by those in the art, is mechanically impossible.
Consequently, while the first (or vertical) axis of the tracking system is rotating from azimuth 90° to azimuth 270°, a period without effective tracking can occur. In the example of a solar power generator, this can lead to a loss in power output.
It must also be borne in mind that altitude-azimuth tracking systems have motors adapted for their application, and hence generally have limited power and therefore speed. For a solar tracking system, these motors are designed to drive the two axes relatively slowly, and it would typically be necessary to employ more powerful motors if it were desired to compensate for the above described problem by driving the tracking system at a faster rate than usual and thereby minimising any tracking delay.
As will also be appreciated, this problem does not arise if an equatorial or polar mount is employed, but altitude-azimuth mounts have advantages (in terms of cost, and ease of construction and erection) that make them highly desirable and widely used.
If this problem is experienced, accurate tracking can recommence after the delay caused by this effect, and the delay (in which data for energy collection is interrupted or reduced) depends on the maximum speed with which the tracking system can switch azimuth from 90° to 270°.
For example, if the maximum azimuth tracking speed is 38° per minute, then to drive the azimuth from 90° to 270° (i.e. by 180°) would take 180° divided by 38° per minute, or 4.74 minutes. If the sun is being tracked, over the course of 4.74 minutes the sun will have moved 1.18°. A zone of 1.18° diameter, centred on the vertical tracking axis projected on the sky, will thus have been either lost or have afforded reduced energy collection.
Owing to the sun's seasonal motion, which is approximately sinusoidal, it dwells longer at the tropics than at the equator. Consequently, for a solar tracking system located on or near the tropics of Capricorn and Cancer, this problem can occur over a series of days around the solstices. On the equator, the problem should occur over fewer days, around the equinoxes.
FIG. 2 illustrates the problem for a solar tracking system with the above characteristics located on the equator. This figure (and FIGS. 3, 4, 5, 7 and 8) are polar diagrams of the sky with the zenith at the centre and the horizon at the circumference 28, with north (N) at the bottom. South (S), east (E) and west (W) are also indicated, as are the sun's track 30 on 21 December (the northern hemisphere winter solstice) at declination −23.5°, the sun's track 32 on 21 June (the northern hemisphere summer solstice) at declination +23.5°, and the sun's track 34 on 21 March and 21 September (the equinoxes) at declination 0°.
The above mentioned zone of 1.18° diameter, above the vertical tracking axis, is indicated (though not to scale) at 36. As can be seen from this figure, this zone 36 is located (for a solar tracking system at latitude 0°) on the celestial equator, and hence is entered by the sun around noon on and around the time of the equinoxes.
For the same solar tracking system located at latitude 23.5° south (such as Alice Springs, in the Northern Territory, Australia), the situation is as depicted in FIG. 3. Zone 36 is located at declination −23.5°, so the sun passes through zone 36 when on solar track 30, that is, on and around the northern hemisphere winter solstice (or southern hemisphere summer solstice) around 21 December.
Thus, this zone 36 is centred on or between the declination −23.5° and +23.5°, with the worst case (in this scenario) occurring when the outer edge of the zone 36 falls on or near these extremes. This occurs, again for this example, when the centre of zone 36 is located at declination (23.5−1.18/2)=±22.91°. Because the sun dwells around the tropics, it will pass into the declination of this zone 36 as it approaches the tropics, and back into that declination, so that there may be weeks around the solstice when the sun passes through zone 36 and this tracking problem arises. This situation is depicted for a tracking system located at latitude 22.91° south in FIG. 4.
Finally, for the same solar tracking system located outside the tropics, the problem does not occur. Thus, for the same solar tracking system located at Melbourne, Victoria, Australia (i.e. latitude 37.5° south), the sun never approaches zone 36 (which lies at declination—37.5°), as my be seen from FIG. 5.